From international pal to Twitter challenger, French President Emmanuel Macron has tweeted a sharp warning to U.S. President Donald Trump over tariffs ahead of the G7 summit in Quebec this weekend.

Trump fired back — and the White House quickly announced that he’s leaving the summit early Saturday before sessions on climate change and the environment.

Leaders are reportedly considering excluding the U.S. from the traditional joint statement issued each year at the end of the G7 summit as a rebuke for Trump’s punishing tariffs against the American allies.

The “American president” doesn’t seem to “mind being isolated,” Macron tweeted Thursday. But “neither do we mind signing a 6 country agreement if need be.” The remaining countries “represent values” and a “true international market force,” he added.

The American President may not mind being isolated, but neither do we mind signing a 6 country agreement if need be. Because these 6 countries represent values, they represent an economic market which has the weight of history behind it and which is now a true international force https://t.co/UA86fcjozs

He added: “The six countries of the G7 without the United States are a bigger market taken together than the American market. We don’t mind being six, if needs be.”

Trump slammed back that France and Canada are hurting the U.S. with unfair trade practices and taunted, “Look forward to seeing them tomorrow.”

Please tell Prime Minister Trudeau and President Macron that they are charging the U.S. massive tariffs and create non-monetary barriers. The EU trade surplus with the U.S. is $151 Billion, and Canada keeps our farmers and others out. Look forward to seeing them tomorrow.

Prime Minister Trudeau is being so indignant, bringing up the relationship that the U.S. and Canada had over the many years and all sorts of other things...but he doesn’t bring up the fact that they charge us up to 300% on dairy — hurting our Farmers, killing our Agriculture!

Trump’s meeting in Quebec is expected to be especially contentious with Macron and Canadian Prime Minister Justin Trudeau, even though the leaders of both nations seemed to have a rapport with Trump and enjoyed visits to the White House — Trudeau last year and Macron in April. But Trump reportedly had angry phone calls with both men as well as with British Prime Minister Theresa May over the tariffs. Trump erroneously accused Canada of burning down the White House in the War of 1812 when he talked to Trudeau.

Trump has unilaterally enacted the tariffs based on “national security.” Sen. Bob Corker (R-Tenn.) introduced legislation Wednesday that would block the president from imposing tariffs on national security grounds without congressional approval.

Trump is turning traditional alliances on their head. While European relationships appear to be fraying, the president is expending special efforts with China and North Korea. Earlier Thursday, the White House announced a deal with China to lift sanctions against its massive technology company ZTE, allowing it to do business again with U.S. firms — despite Congress’ concerns about national security risks. And he’s preparing to meet with North Korean leader Kim Jong Un in Singapore on Tuesday.

“The traditional definition of allies is certainly being called into question,” Douglas Rediker, executive chairman of the financial advisory firm International Capital Strategies, told The Washington Post. “If you look at it holistically, then it doesn’t make sense.” But he said it’s “consistent” for a president driven by the “political imperative of being seen as cutting a big deal on a big issue.”

House Minority Leader Nancy Pelosi (D-Calif.) complained that “China is eating our lunch, and President Trump is serving it to them.”